In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC (alternate current) motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC (direct current) power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and the DC voltage of the DC power source is converted into AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in the hybrid automobile and the electric automobile like this, there is known a technology in which by disposing a current sensor for sensing a phase current only in one phase, the number of current sensors is decreased and a part near three-phase output terminals of an inverter is reduced in dimension and the cost of a control system of the AC motor is reduced (see, for example, patent document 1). Further, as a technology for controlling an electric motor on the basis of a current sensed value of only one phase, for example, as shown by patent document 2, there is also proposed a technology in which in order to prevent a torque ripple by a three phase unbalance caused by a gain error between current detectors, an ideal AC waveform based on a current value of one phase is acquired and the electric motor is controlled by the ideal AC waveform.
In the patent document 2, from the fact that an ideal AC waveform has its respective phases shifted by 120° from each other, the current values of the other phases are estimated by simply delaying the current value of one phase by 120° so as to match the ideal AC waveform. By the method like this, essentially, two-dimensional quantities (for example, a d-axis current and a q-axis current) necessary for a vector control cannot be correctly controlled.
Further, in the patent document 1, the other phase current is estimated by multiplying a sensed value of the current sensor of one phase by a coefficient, but not only in a locked state but also in the state where the sensed value becomes 0, the current values of all phases become 0. Further, since the three-phase alternating currents are assumed to be ideal sine waves and a phase and a coefficient are derived from the ideal sine waves, it is impossible to acquire estimated values reflecting actual currents.
For example, in order to perform a vector control with high accuracy in a system in which a main motor needs to be performed at high speed and with high accuracy, as is the case of the main motor of the hybrid vehicle and the electric automobile, it is necessary to make a correction for adding a remaining one-dimensional component to the current sensed value of one phase. To that end, it is considered to estimate two-dimensional quantities with high accuracy by the use of information changed in synchronization with the rotation of an AC motor. At this time, when the number of revolutions of the AC motor is small, a change in the information changed in synchronization with the rotation of the AC motor is small and hence the accuracy of current estimation is likely to be impaired.
When the accuracy of current estimation is impaired, an abnormal current is likely to be passed through the AC motor to thereby generate an abnormal torque. When the abnormal torque is generated, for example, in the case where the AC motor is mounted in the hybrid vehicle and the electric automobile, drivability will be impaired. Here, the abnormal current is assumed to include a too small current and a too large current. Further, the abnormal torque is assumed to include a too small torque by a too small current and a too large torque by a too large current.
For example, in the case where the AC motor is applied to the main motor of the hybrid vehicle or the electric automobile, the following problem can be caused: that is, when a too small current is passed through the AC motor, a desired torque is not generated; and hence, a user is likely to depress an accelerator pedal more than necessary to suddenly generate toque, or when the accuracy of current estimation is improved, torque is likely to be suddenly generated, which hence can cause a sudden start.
Further, when a too large current is passed through the AC motor, a various kinds of elements used for the control device of the AC motor are likely to generate heat or to cause a failure.
[Patent document 1] Japanese Unexamined Patent Publication No. 2004-159398
[Patent document 2] Japanese Patent No. 4942425